The commercial production of mushrooms (Agaricus bisporus) involves a series of steps, including compost preparation, compost pasteurization, inoculating the compost with the mushroom fungus (spawning), incubation to allow thorough colonization of the compost with mushroom mycelia, top dressing the compost with moistened peat moss (casing), and controlling the environment to promote the development of mature mushrooms. The mushroom growing process is described in detail in several publications (for example, Chang & Hayes, 1978; Flegg et al., 1985; Chang & Miles, 1989; Van Griensven, 1988).
Mushroom spawn is used to inoculate the nutritive substrate (compost). Virtually all spawn now used is based on a grain substrate. The technology for making grain based mushroom spawn was first taught by Sinden (U.S. Pat. No. 1,869,517). Spawn is generally made from sterilized grain that is inoculated with pure cultures of the desired mushroom strain. Mushroom spawn can be prepared by several methods. In one method, dry grain (rye, millet, wheat, sorghum, or other grain), water, CaCO.sub.3, and (optionally) CaSO.sub.4 are placed in suitable containers and capped with lids that allow passage of air and steam but do not allow the passage of microbes that would contaminate the finished product. Containers are subject to steam sterilization for times and temperatures suitable to render the mixtures commercially sterile. Following cooling, the grain mixture is inoculated with a starter culture of the desired mushroom strain, and incubated under permissive conditions for approximately 14 days. Containers are shaken at specific intervals to promote even colonization of the mycelium throughout the mixture. Following complete colonization of the hydrated, sterile grain with the mushroom fungus, the spawn can be used immediately to inoculate mushroom compost. The mixtures can also be transferred to plastic bags and refrigerated in anticipation of spawning at a future date.
Spawn properly prepared according to the above cited method has the following characteristics: Approximately 48 to 50 wt % moisture, pH 6.6 to 7.2, free flowing, even white color resulting from the heavy growth of the Agaricus bisporus mycelium. Spawn is generally added to mushroom compost at a rate of 2-4% (fresh weight spawn/dry weight compost). Since rye spawn contains about 1.15% nitrogen (Kjeldahl) on a fresh weight basis (about 2.3% on a dry weight basis), and also contains carbohydrate and lipid, spawn contributes some nutrients to the mushroom substrate.
Properly prepared mushroom spawn is resistant to contamination by foreign microorganisms. The heavy growth of the mushroom mycelium on the grain particles excludes the growth of many competitor microorganisms. Even when spawn is added to mushroom compost, which contains high levels of bacteria and molds, properly prepared spawn does not show overt growth of foreign microorganisms (Elliott, 1985). This is in part due to the exclusionary effect of the heavy growth of the Agaricus bisporus mycelium and in part due to the "selectivity" of properly prepared mushroom compost.
An alternate method of spawn production involves bulk cooking of grain in large kettles. Grain and water mixtures are heated to hydrate the grain. After draining excess water, the hydrated grain is mixed with CaCO.sub.3 and CaSO.sub.4, filled into bottles or heat resistant plastic bags, sterilized, cooled, inoculated with starter cultures of the desired mushroom strain, and incubated to allow colonization of the grain with the mycelium.
Another method of spawn production involves placing grain, water, CaCO.sub.3, and CaSO.sub.4 into steam jacketed mixers. Mixtures are cooked, sterilized, cooled, and inoculated in the mixers. The inoculated sterile grain is aseptically transferred to sterile plastic bags that are ventilated to allow passage of air while maintaining sterility. Following mycelial growth, spawn can be shipped to mushroom production facilities with minimal further handling of the product.
Virtually all spawn used to inoculate mushroom compost is made using rye, millet, wheat, sorghum, or other grain substrate. Fritsche (1978) describes a formula reported by Lemke (1971) for spawn on a perlite substrate. The formula is as follows: perlite (1450 g), wheat bran (1650 g), CaSO.sub.4.2 H.sub.2 O (200 g), CaCO.sub.3 (50 g), water (6650 ml). The pH after sterilization is 6.2 to 6.4. This formula is calculated to contain 1.10 to 1.34% nitrogen on a dry weight basis (assuming a typical nitrogen content of wheat bran of 2.24 to 2.72%).
Stoller (U.S. Pat. No. 3,828,470) teaches that mushroom mycelium will not grow on feedstuffs such as cottonseed meal, soybean meal, etc., when used alone as an autoclaved substrate. Stoller also teaches spawn in which the cereal substrate has been diluted with an inorganic material containing calcium carbonate or an organic flocculating agent. Nitrogen contents are generally low. For example, Stoller's example 16 is estimated to contain about 0.22% nitrogen. Stoller's example 18 is estimated to contain about 0.7% nitrogen. Stoller also teaches that a fine, granular or powdery spawn is preferable to the large, whole grain particles of grain spawn. This is generally due to the number of "points of inoculum" per unit weight of spawn.
Romaine (U.S. Pat. No. 4,803,800) teaches production of mushroom casing spawn by encapsulation of nutrients in a hydrogel polymer. Casing spawn is used to inoculate the mushroom casing layer rather than the compost. Use of casing spawn speeds fruiting. Nitrogen contents in the Romaine casing spawn are generally low. For example, Romaine teaches total nutrient levels of 2 to 6% (wt/vol of formula). Assuming the use of 100% protein as the nutrient source, total nitrogen would be about 0.96%. Some of Romaine's formulas contain perlite, vermiculite, soy grits, or similar materials at about 2 to 6% (wt/vol) of the formula as texturizing agents.
Dahlberg & LaPolt (U.S. Pat. No. 5,503,647) teach the development of a mushroom casing spawn prepared from nutritionally inert particles (calcined earth, vermiculite, perlite, etc) amended with nutrients. The casing spawn is formulated with low nitrogen contents (generally less than 1%) to allow inoculation of the mushroom casing layer with Agaricus bisporus mycelium without promoting the growth of pests and pathogens. Dahlberg & LaPolt also teach that high levels of proteinaceous ingredients such as soybean fines, etc. are inhibitory to Agaricus bisporus growth. Generally, nitrogen levels above about 2% in a casing spawn formula result in reduced growth of Agaricus bisporus mycelium. This casing spawn formulation is also proposed as a substrate for inoculation of spawn during its preparation.
Mushroom Supplements:
Many mushroom growers add nutrient supplements to the mushroom compost at the time of spawning or casing. Because of the danger of spreading diseases, especially at tray-type mushroom farms, most mushroom growers add supplements at spawning. Addition of such supplements usually results in an increase in mushroom yield. Nutrient supplements generally consist of proteinaceous materials such as cracked soybean particles, soybean meal, corn gluten, feather meal, and similar materials. For example, in Hughes et al. (U.S. Pat. No. 3,560,190), a dry formulation based on a combination of cottonseed meal and cottonseed oil is disclosed as a suitable supplement.
Nutrient supplementation, however, is susceptible to some undesirable effects. One problem that has been encountered is excessive bed heating, apparently caused by the ready availability of the nutrient source to the highly active microbial mushroom culture. Temperature excursions above 35.degree. C. (95.degree. F.), sufficient to significantly deplete, if not completely destroy the mushroom mycelia have been observed. Another problem is encountered when adding the supplement to the compost at the time of spawning. In many cases, other microorganisms, primarily molds, preexisting in the compost, introduced with the supplement, or introduced via airborne contamination, compete with the mushroom mycelium for the added nutrients. This reduces the availability of the supplement for its intended purpose, and often hinders the development of the mushroom mycelium.
Recognizing these problems, Carroll et al. (U.S. Pat. No. 3,942,969) provides a supplement suitable for addition to the compost at the time of spawning, in which the release of the nutrient is delayed. The supplement comprises a denatured protein source, including protein derived from cottonseed, soybean, and peanuts. As disclosed, the denaturing can be accomplished by heat treating or by treatment with alkalies, acids, or formaldehyde. Unfortunately, the potential gains in mushroom yields are disadvantageously offset by the economic penalty associated with the denaturation treatment. The potential health and environmental hazards of denaturing treatments such as formaldehyde is also a disadvantage.
Wu (U.S. Pat. No. 4,534,781) teaches an improved nutrient supplement comprising a particulate nutrient, such as a cracked soybean particle, coated with a hydrophobic material that is not readily assimilable by competing microorganisms in the compost. A further improvement in this technology was taught by Wu & Bretzloff (U.S. Pat. No. 4,617,047) in which the protein containing nutrient is coated with a hydrophobic material and a mold inhibitory composition. Again, the potential gains in mushroom yield are disadvantageously offset by the cost associated with the antimicrobial treatments. The cost and potential health and environmental hazards of the mold inhibitory treatments are also a disadvantage.
Katz et al. (Eur. Pat. Publ. 0 0290 236) teaches another nutrient supplement for mushroom cultivation, prepared by coating protein rich particles with a hydrophilic carbohydrate. This coating also retards the release of nitrogen into the medium. Pratt et al. (U.S. Pat. No. 4,764,199) teach a mushroom growing supplement prepared from acidic corn gluten meal treated with aqueous formaldehyde while maintaining the meal in a free flowing condition.
Romaine & Marlowe (U.S. Pat. Nos. 5,291,685 and 5,427,592) teach another nutrient supplement for mushroom cultivation in which intact seeds, such a rapeseed, or other small oilseed are heat treated, such as at 90.5.degree. C. (195.degree. F.) for 24 hours. The heat treatment prevents sprouting and provides a delayed release mechanism for seed nutrients. The Romaine & Marlowe supplement is used at fairly high rates of between 7 and 14% of the dry weight of the compost All prior art for mushroom supplements involves treating nutrients with heat or chemicals to reduce the availability of the nutrients to competing microorganisms in the compost. In all cases, treatments represent a significant portion of the cost of the supplement. In the case of chemical treatments of the supplements, ingredients such as formaldehyde and various pesticides represent potential health and environmental hazards, and the practicality of using such agents may be reduced due to regulatory issues. The development of mushroom supplements without using such chemicals is highly desirable.
Brini & Sartor (European Patent Application EP 0 700 884 A1) teach a mixture of a water retaining-dispersing agent (e.g., peat), a buffer, a protein containing component (e.g. soybean meal), a growth promoting component (e.g. corn gluten and/or corn starch), and water. The mixture is sterilized, inoculated with the mushroom fungus, and used to spawn mushroom compost. The formulation inoculates the mushroom beds and adds protein, while eliminating residual antimicrobial substances and suppressing the growth of antagonistic molds. Moisture contents of the mixtures are typically 54 to 60%. Protein contents of the mixtures are 4 to 20 wt % protein (1.4 to 8.0 wt % nitrogen). Mixtures typically contain 7.4 to 15.2 wt % (2.63 to 6.08 wt % nitrogen). Use of the mixtures as mushroom spawn is asserted to allow the faster growth of the mushroom and prevent the growth of molds. However, routine experimentation has shown that the mixtures taught by Brini & Sartor tend to form clumps, resulting in incomplete sterilization and areas within the mixtures that are not completely colonized by the Agaricus bisporus mycelium. The failure to achieve sterilization results in an economic loss, while a poorly colonized mixture can allow the growth of competitor molds and bacteria in the compost, causing high compost temperatures and reducing mushroom yield.